Ridiculously slow internet

Heres the situation. We have 3 computers in our house, a wired desktop (mine), a wireless one and a wireless laptop.
On my PC i have no trouble with speeds, nor does the laptop, the wireless desktop however, at times barely works at all.
It is running XP with an Edimax EW-7728IN / PCI Wireless N WiFi Card / 300mbps / 802.11n, now we recently bought this card to resolve the slow internet problems, did not work.
on speedtest.net it gets less than 1mb download normally with about 150ms ping, and when we should get 20mb this is fairly frustrating, I would like to know a way of resolving this problem without having to mess aroung with wires through the house.

 

How good a connection do you get? also what is the connection speed of nic for the wireless conection

 

From the router the connection is fairly good, around 18-20mb.
Also when pinged through cmd prompt it averages 12ms.

 

Hello, approximately what is the distance between the wireless router and wireless desktop?

Dacads

 

About 10 meters at the very most, with only a thin wall in the way. This computer is essentially in a shed.

 

go to the properties of the wireless card and set it to autonegotiate